Automated Non-Destructive Inspection of Surface Skins Using Transporter System

ABSTRACT

Systems and methods for automated maintenance of the top and bottom surfaces or skins of an integrally stiffened hollow structure (e.g., a horizontal stabilizer) using surface crawling vehicles. Each system uses dynamically controlled magnetic coupling to couple an external drive tractor to a pair of passive trailers disposed in the interior of the hollow structure on opposite sides of a vertical structural element. The external drive tractor is also coupled to an external maintenance tool, which the tractor pushes or pulls across the surface skin to perform a maintenance function. The systems allow maintenance operations to be performed on both surface skins without turning the hollow structure over. Each system is modular and can be transported to and easily set up in a building or factory.

RELATED PATENT APPLICATIONS

This application is a continuation of and claims priority from U.S.patent application Ser. No. 13/859,278 filed on Apr. 9, 2013, which inturn is a continuation-in-part of and claims priority from U.S. patentapplication Ser. No. 13/534,014 filed on Jun. 27, 2012, which issued asU.S. Pat. No. 9,010,684 on Apr. 21, 2015.

BACKGROUND

The present disclosure relates generally to the field of automatedmaintenance (including non-destructive inspection) of aircraftstructural elements, and more particularly to an automated endeffector-carrying apparatus that is coupled to and travels along anintegrally stiffened wing box while performing a maintenance function.As used herein, the term “maintenance” includes, but is not limited to,operations such as non-destructive inspection (NDI), visual inspection,drilling, scarfing, grinding (e.g., to remove bonded or boltedcomponents), fastening, appliqué application, ply mapping, depainting,cleaning and painting.

A variety of elongated composite structures may have relatively confinedinternal cavities that require inspection in order to assure that thestructure meets production and/or performance specifications. One knownelongated composite structure with tapering internal cavities is anintegrally stiffened wing box for an airplane. One example applicationis in the horizontal stabilizer of an aircraft. A horizontal stabilizerstructural box may be fabricated as a large co-cured structure thatrequires the use of soft internal tools to facilitate tool removal afterthe cure. If a given co-cured composite structure is considered primarystructure, it would therefore need to be inspected to ensure structuralintegrity.

One of the requirements for certification of a composite-based airplanehorizontal stabilizer is to perform a complete set of NDI scan of allthe composite structural elements. Methods for scanning the interiorsurfaces of a horizontal stabilizer using a modular, magneticallycoupled transporter system have been disclosed, for example, in U.S.patent application Ser. No. 13/534,014. With regard to the exteriorsurfaces of a horizontal stabilizer, it is known to use a large gantrymechanism that moves an ultrasonic maintenance tool over a surface skin.This gantry-based system is expensive, requires extensive training tooperate, and occupies a large space, which limits the flexibility insetting up NDI work cells for scanning of horizontal stabilizers It alsorequires that the horizontal stabilizer be turned over to scan theopposite surface.

It would be desirable if a process were available that could scan wingbox surface skins without the need for a gantry-based mechanism formovement of the NDI sensor array. An additional benefit would be if thesurface skin scanning process were compatible with the aforementionedprocess for scanning the interior surfaces. Accordingly, there is a needfor a system for inspecting the exterior of a wing box and similarelongated hollow structures that can provide such benefits.

SUMMARY

The subject matter disclosed herein includes systems and methods forautomated NDI scanning of the top and bottom aerodynamic surfaces orskins of an integrally stiffened wing box (e.g., a horizontalstabilizer) using surface crawling vehicles. In accordance with variousembodiments disclosed herein, the system uses dynamically controlledmagnetic coupling to couple an external drive tractor to a pair ofpassive trailers disposed inside a wing box on opposite sides of a spar.The externally mounted drive tractor is also coupled to an externallymounted payload platform, which the tractor pushes or pulls across thesurface skin being inspected. The disclosed systems allow scanning ofboth surface skins without turning the integrally stiffened wing boxover. Each system is modular and can be transported to and easily set upin a building or factory.

One aspect of the subject matter disclosed herein is a method forscanning a wing box skin, comprising: (a) placing a first tractorvehicle in a position external to the wing box and in contact with theskin; (b) placing first and second trailer vehicles in respectiveinterior spaces of the wing box with a first spar of the wing boxtherebetween; (c) magnetically coupling the first and second trailervehicles to the first tractor vehicle with the skin therebetween and toeach other with the first spar therebetween; (d) coupling a payloadplatform to the first tractor vehicle in a position external to the wingbox, the payload platform comprising a frame and a maintenance tool thatis movable relative to the frame; (e) moving the first tractor vehiclealong a path that follows the first spar; (f) stopping the first tractorvehicle; and (g) moving the maintenance tool of the payload platform ina first direction relative to the frame of the payload platform whilethe first tractor vehicle is stopped in step (f). The foregoing methodmay further comprise: (h) placing a second tractor vehicle in a positionexternal to the wing box and in contact with the skin; (i) placing thirdand fourth trailer vehicles in respective interior spaces of the wingbox with a second spar of the wing box therebetween; (j) magneticallycoupling the third and fourth trailer vehicles to the second tractorvehicle with the skin therebetween and to each other with the secondspar therebetween; (k) coupling the payload platform to the secondtractor vehicle; (l) during step (e), moving the second tractor vehiclealong a path that follows the second spar; and (m) stopping the secondtractor vehicle, wherein step (g) is performed while the first andsecond tractor vehicles are not moving. The maintenance tool can be aninspection unit that transmits beams toward the skin and receivesreflection signals returned to the inspection unit receiver.

In accordance with another aspect, the scanning method set forth in thepreceding paragraph may further comprise the following steps: placingfirst, second and third wing box support tools under the wing box, thefirst wing box support tool being closer to a root end of the wing boxthan is the second wing box support tool and the third wing box supporttool being closer to a tip end of the wing box than is the second wingbox support tool, each of the first, second and third wing box supporttools being configurable between a first state wherein it supports thewing box and obstructs the payload platform and a second state whereinit neither supports the wing box nor obstructs the payload platform;configuring the first, second and third wing box support tools so thatthe second and third wing box support tools support the wing box whilethe first wing box support tools does not; while the second and thirdwing box support tools are supporting the wing box, moving the firsttractor vehicle from a position whereat the payload platform overlies aspace between the root end of the wing box and the first wing boxsupport tool to a position whereat the payload platform overlies a spacebetween the first and second wing box support tools; after the precedingstep has been performed, reconfiguring the first and second wing boxsupport tools so that the first and third wing box support tools supportthe wing box while the second wing box support tools does not; and whilethe first and third wing box support tools are supporting the wing box,moving the first tractor vehicle from the position whereat the payloadplatform overlies a space between the first and second wing box supporttools to a position whereat the payload platform overlies a spacebetween the second and third wing box support tools. A fourth wing boxsupport tool can be employed to facilitate passage of the payloadplatform from one side to the other side of the third wing box supporttool.

A further aspect of the subject matter disclosed herein is an apparatusfor scanning a wing box skin, comprising: a first tractor vehiclecomprising a first frame, a plurality of wheels rotatably coupled to thefirst frame, a first coupling element, a first plurality of magnetssupported by the first frame, a first drive wheel for driving the firsttractor vehicle to move, and a first motor for driving rotation of thefirst drive wheel, the first motor being supported by the first frame;and a first payload platform comprising a second frame, a plurality ofwheels rotatably coupled to the second frame, a second coupling element,a first maintenance tool supported by and movable relative to the secondframe, and a first actuator for moving the first maintenance toolrelative to the second frame, the first actuator being supported by thesecond frame, wherein first and second coupling elements are coupled toeach other.

In accordance with one embodiment, the apparatus described in thepreceding paragraph may further comprise a second tractor vehicle, thesecond tractor vehicle comprising a third frame, a plurality of wheelsrotatably coupled to the third frame, a third coupling element, a secondplurality of magnets supported by the third frame, a second drive wheelfor driving the second tractor vehicle to move, and a second motor fordriving rotation of the second drive wheel, the second motor beingsupported by the second frame, wherein the first payload platformfurther comprises a fourth coupling element, the third and fourthcoupling elements being coupled to each other. The first couplingelement is pivotable relative to the second coupling element, and thethird coupling element is pivotable and slidable relative to the fourthcoupling element.

In accordance with another embodiment, the apparatus described twoparagraphs above may further comprise a second payload platformcomprising a third frame, a plurality of wheels rotatably coupled to thethird frame, a third coupling element, a second maintenance toolsupported by and movable relative to the third frame, and a secondactuator for moving the second maintenance tool relative to the thirdframe, the second actuator being supported by the third frame, whereinthe first tractor vehicle further comprises a fourth coupling element,the third and fourth coupling elements being coupled to each other.

A further aspect of the subject matter disclosed herein is a system forperforming a maintenance function on a wing box skin, comprising: (a) ahollow composite structure comprising first and second spars and firstand second skins connected by the first and second spars; (b) a mobileplatform comprising: (i) a chassis comprising first and second chassisparts coupled to each other, the first chassis part overlying a firstportion of the first spar, each of the first and second chassis partscomprising a respective plurality of wheels in contact with the externalsurface of the first skin; (ii) a first drive wheel rotatably coupled tothe first chassis part and in contact with the external surface of thefirst skin; (iii) a first actuator mounted to the first chassis part forcausing the first drive wheel to rotate; (iv) a first plurality ofmagnets mounted to the first chassis part; and (v) a first maintenancetool slidably coupled to the second chassis part, the first maintenancetool being slidable along the second chassis part; and (vi) a secondactuator mounted to the second chassis part for causing the firstmaintenance tool to slide along the second chassis part; (c) a firsttrailer vehicle disposed adjacent a first portion of an internal surfaceof the first skin and adjacent one side of the first spar, the firsttrailer vehicle comprising a second plurality of magnets, at least onemagnet pole of the second plurality of magnets being magneticallycoupled to a magnet pole of the first plurality of magnets through thefirst skin; and (d) a second trailer vehicle disposed adjacent a secondportion of an internal surface of the first skin and adjacent anotherside of the first spar, the second trailer vehicle comprising a thirdplurality of magnets, at least one magnet pole of the third plurality ofmagnets being magnetically coupled to a magnet pole of the firstplurality of magnets through the first skin, and at least one magnetpole of the third plurality of magnets being magnetically coupled to amagnet pole of the second plurality of magnets through the first spar,wherein the magnetically coupled mobile platform and first and secondtrailer vehicles move in unison when the drive wheel is rotated.

In cases where the first maintenance tool is an inspection unit, themobile platform may further comprise means for measuring an X positionand a Y position of the inspection unit, and the system furthercomprises a pulser/receiver unit operatively coupled to the inspectionunit and to the first and second encoding means. The pulser/receiverunit is programmed to perform the following operations: sending controlsignals to the inspection unit; receiving scan data signals from theinspection unit; receiving X-Y position data signals from the measuringmeans; and correlating the scan data with the X-Y position data.

In accordance with another aspect, the system may further comprise: aplurality of motion script files containing sequences of motion commandsand parameters respectively associated with a plurality of motion paths;and a computer system programmed to execute a sequence of commands in aselected one of plurality of motion scripts, the sequence of commandscontrolling operation of the first and second actuators to cause thefirst maintenance tool to move along a corresponding selected one of themotion paths in accordance with its associated parameters.

In accordance with one embodiment, the chassis further comprises a thirdchassis part coupled to the second chassis part, the third chassis partcomprising a respective plurality of wheels in contact with the externalsurface of the first skin, the third chassis part overlying a portion ofthe second spar. In this embodiment, the mobile platform furthercomprises: a second drive wheel rotatably coupled to the third chassispart and in contact with the external surface of the first skin, thethird chassis part being movable along a third motion path when thesecond drive wheel rotates while in contact with the external surface ofthe first skin; a second actuator mounted to the third chassis part forcausing the second drive wheel to rotate; a fourth plurality of magnetsmounted to the third chassis part; a third encoder for measuring aposition of the third chassis part along the third motion path. Inaddition, the system further comprises: a third trailer vehicle disposedadjacent a third portion of an internal surface of the first skin andadjacent one side of the second spar, the third trailer vehiclecomprising a fifth plurality of magnets, at least one magnet pole of thefifth plurality of magnets being magnetically coupled to a magnet poleof the fourth plurality of magnets through the first skin; and a fourthtrailer vehicle disposed adjacent a fourth portion of an internalsurface of the first skin and adjacent another side of the second spar,the second trailer vehicle comprising a sixth plurality of magnets, atleast one magnet pole of the sixth plurality of magnets beingmagnetically coupled to a magnet pole of the fourth plurality of magnetsthrough the first skin, and at least one magnet pole of the sixthplurality of magnets being magnetically coupled to a magnet pole of thefifth plurality of magnets through the second spar. In this embodiment,the first chassis part is pivotably coupled to the second chassis part,and the third chassis part is pivotably coupled and slidably coupled tothe second chassis part.

In accordance with an alternative embodiment, the chassis furthercomprises a third chassis part coupled to the first chassis part, thethird chassis part comprising a respective plurality of wheels incontact with the external surface of the first skin, the third chassispart overlying a second portion of the first spar. In this alternativeembodiment, the mobile platform further comprises: a second maintenancetool slidably coupled to the third chassis part, the second maintenancetool being slidable along the third chassis part; and a third actuatormounted to the third chassis part for causing the second maintenancetool to slide along the third chassis part.

Other aspects are disclosed and claimed below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing an orthographic view of a portion of ageneralized horizontal stabilizer of an airplane having top and bottomskins or panels connected by a plurality of spars.

FIG. 2 is a diagram showing side views of a tractor-trailerconfiguration having means for adaptive magnetic coupling. A secondtrailer vehicle is not visible. The left-hand side of FIG. 2 shows aninspection scenario wherein the trailer vehicles are inverted, while theright-hand side shows an inspection scenario wherein the tractor vehicleis inverted.

FIG. 3 is a diagram showing an end view of the tractor-trailerconfiguration depicted on the left-hand side of FIG. 2 (with respectiveinverted trailer vehicles disposed on opposing sides of a spar).

FIG. 4 is a diagram showing a top view of an external skin scanningapparatus atop a surface skin of a wing box, the apparatus having adouble-tractor configuration in accordance with one embodiment. Theapparatus is shown in two possible positions.

FIG. 5 is a diagram showing a top view of a wing box skin with apotential scan path plan (assuming installation of a runoff component atthe wide end of the wing box) depicted as a serpentine line superimposedon the skin, which serpentine line represents a path of travel of apoint on an NDI sensor during scanning of the skin using the apparatusdepicted in FIG. 4.

FIG. 6 is a diagram showing a top view of an external skin scanningapparatus atop a surface skin of a wing box, the apparatus having adouble-tractor configuration in accordance with another embodiment. Theapparatus is shown in two possible positions.

FIG. 7 is a diagram showing a top view of a wing box skin with apotential scan path plan (assuming installation of a runoff component atthe wide end of the wing box) depicted as a serpentine line superimposedon the skin, which serpentine line represents a path of travel of apoint on an NDI sensor during scanning of the skin using the apparatusdepicted in FIG. 6.

FIG. 8 is a diagram showing a top view of the external skin scanningapparatus depicted in FIG. 6, wherein the trailer vehicles are supportedby a run-on component attached to the root end of the wing box.

FIG. 9 is a diagram showing a top view of an external skin scanningapparatus atop a surface skin, the apparatus comprising two tractorspushing/pulling two crossbar bridges carrying respective NDI sensorunits in accordance with a further embodiment.

FIG. 10 is a diagram showing a top view of an external skin scanningapparatus atop a surface skin of a wing box, the apparatus comprising asingle tractor coupled to a single NDI scanner trailer in accordancewith yet another embodiment. The apparatus is shown in two possiblepositions and two possible operational modes (i.e., push and pullmodes).

FIGS. 10A and 10B are diagrams showing side views of a configuration inwhich a scanning apparatus of the type shown in FIG. 10 is magneticallycoupled to respective sets of passive trailer vehicles disposed onopposing sides of a spar and magnetically coupled to each other. Thesecond set of passive trailer vehicles (disposed behind spar web 8) isnot visible in either FIG. 10A or FIG. 10B. FIG. 10A shows an inspectionscenario wherein the scanning apparatus is atop the top skin, while FIG.10B shows an inspection scenario wherein the scanning apparatus isinverted and underneath the bottom skin.

FIG. 11 is a diagram showing a top view of a wing box skin with apotential scan path plan (assuming installation of a runoff component atthe wide end of the wing box) depicted as a serpentine line superimposedon the skin, which serpentine line represents a path of travel of apoint on an NDI sensor during scanning of the skin using the apparatusdepicted on the left-hand side of FIG. 10.

FIG. 12 is a diagram showing a top view of a wing box skin with apotential scan path plan (assuming installation of a runoff component atthe wide end of the wing box) depicted as a serpentine line superimposedon the skin, which serpentine line represents a path of travel of apoint on an NDI sensor during scanning of the skin using an apparatusthat differs from that depicted on the left-hand side of FIG. 10 in thatthe scan plane of the NDI sensor unit is oriented parallel to thedirection of tractor travel instead of perpendicular thereto.

FIG. 13 is a diagram showing a top view of an external skin scanningapparatus atop a surface skin of a wing box, the apparatus comprising asingle tractor coupled to front and rear NDI scanner trailers inaccordance with a further embodiment. In this embodiment, the scanplanes of the NDI sensor units are oriented parallel to and movable(relative to the tractor) in a direction perpendicular to the directionof tractor travel.

FIG. 14 is a diagram showing a top view of an external skin scanningapparatus comprising a single tractor coupled to front and rear NDIscanner trailers in accordance with a further embodiment. In thisembodiment, the NDI sensor units are oriented perpendicular to andmovable (relative to the tractor) in a direction perpendicular to thedirection of tractor travel.

FIGS. 14A and 14B are diagrams showing a side view and an end viewrespectively of a configuration in which a scanning apparatus of thetype shown in FIG. 14 is magnetically coupled to respective sets ofpassive trailer vehicles disposed on opposing sides of a spar andmagnetically coupled to each other. FIGS. 14A and 14B show an inspectionscenario wherein the scanning apparatus is atop the top skin.

FIGS. 15A through 15C are diagrams showing respective side views ofconfigurable tools designed to support a wing box during non-destructiveinspection using the systems disclosed herein. FIGS. 15A through 15Cshow three configurations which may occur during a configuration changesequence.

FIG. 16 is a block diagram showing a control system in accordance withone embodiment.

FIG. 17 is a diagram representing a screen shot of a graphical userinterface for controlling the inspection system disclosed herein.

Reference will hereinafter be made to the drawings in which similarelements in different drawings bear the same reference numerals.

DETAILED DESCRIPTION

The maintenance tool-scanning mobile platform disclosed herein isdesigned for scanning a maintenance tool over an external surface of askin of a hollow structure. As used herein, the term “maintenance tools”includes, but is not limited to, NDI units, drills, scarfers, grinders,fasteners, appliqué applicators, ply mappers, and depainting, cleaningand painting tools. For the purpose of illustration, various embodimentswill be described in which the maintenance tool is an NDI unit (e.g., anarray of ultrasonic transducers).

In accordance with the embodiments disclosed herein, ultrasonic NDIsensors are used to inspect a hollow composite structure, such as anintegrally stiffened wing box for an aircraft (e.g., a horizontalstabilizer). A portion of a generalized integrally stiffened wing box 2for an aircraft is depicted in FIG. 1. The depicted integrally stiffenedwing box comprises a top skin 4 and a bottom skin 6 connected by aplurality of internal vertical support elements, hereinafter referred toas “spars”. Each spar comprises a web 8 and respective pairs of filletedjoin regions 10 (also called “spar radii”), which connect the spar web 8to the top and bottom skins. As used herein, the terms “top skin” and“bottom skin” refer to the relative positions of two skins of a wing boxwhen the wing box is being inspected, not when the wing box is installedon an airplane (i.e., a wing box may be inverted for inspection).

In accordance with the system disclosed in U.S. patent application Ser.No. 13/534,014 (the contents of which are incorporated by referenceherein in their entirety), an NDI sensor (e.g., a linear ultrasonictransducer array) is transported down the length of a tunnel through theinterior of the composite structure. For this type of inspection, thesensor is carried by a trailer vehicle (not shown in FIG. 1) placedinside the hollow structure 2. This trailer vehicle can be characterizedas being “active” in the sense that equipment it carries is activelyperforming a scanning function. For some types of inspectionapplications, the sensor needs to be acoustically coupled to eachsurface being inspected while an automated external tractor vehicle(also not shown in FIG. 1) moves the trailer vehicle along that surfacein a region of interest. In the case of ultrasonic inspection, acousticcoupling is provided by a column of water that flows between the sensorand the inspected part.

In FIG. 1, portions of the interior surfaces of the part which need tobe inspected can be seen. Each spar needs to have web 8 and all fourfilleted join regions 10 inspected. This is a challenging inspection aseach cavity is essentially a long rectangular tunnel that decreases incross section as one moves from root to tip. A system designed for sparinspection using scanning apparatus placed inside the wing box isdisclosed in U.S. patent application Ser. No. 13/534,014. In contrast,this disclosure is directed to inspecting the top and bottom skins 4 and6 using scanning apparatus placed outside the wing box.

The top and bottom skins of a wing box can be inspected by a transportersystem comprising magnetically coupled external and internal vehicles.The basic principle of operation of such magnetically coupled vehicleswill now be described with reference to FIGS. 2 and 3, which side andend views respectively of an external motorized and computer-controlledtractor 12 magnetically coupled to an internal trailer 14 disposedinside a wing box. Also, there is an internal trailer 16 (see FIG. 3) onthe opposite side of the spar that is magnetically coupled through thespar to trailer 14 and also magnetically coupled through the skin to thetractor 12. This three-part system gives a very stable system forpositioning and moving an NDI sensor unit, such as a unit comprising anarray of ultrasonic transducers configured to scan in a scan plane.

FIG. 2 shows side views of a tractor-trailer configuration in accordancewith one embodiment in two different inspection situations (motoractuators are not shown). The automated NDI inspection system comprisesa traction-motor powered tractor vehicle 12, which rides on the externalsurface of top skin 4 or bottom skin 6 of wing box 2, and a pair oftrailer vehicles (only trailer vehicle 14 is visible in FIG. 2, theother being hidden behind a spar web 8), which ride along an internalsurface of the top or bottom skin. The left-hand side of FIG. 2 shows aninspection scenario wherein the tractor vehicle 12 is outside the wingbox in a non-inverted position while the trailer vehicles are inside thewing box in inverted positions; the right-hand side of FIG. 2 shows aninspection scenario wherein the tractor vehicle 12 is outside the wingbox in an inverted position while the trailer vehicles are inside thewing box in non-inverted positions. FIG. 3 shows an end view of thetractor-trailer configuration depicted on the left-hand side of FIG. 2,with inverted trailer vehicles 14 and 16 disposed on opposite sides of aspar.

In the inspection scenario depicted in FIG. 3 (and the left-hand side ofFIG. 2), idler wheels 18 of tractor vehicle 12 contact and roll on theexternal surface of top skin 4 while vertical idler wheels 20 ofinverted trailer vehicles 14 and 16 (only one such idler wheel isvisible in FIG. 3 for each trailer vehicle) contact and roll on theinternal surface of top skin 4, and the horizontal idler wheels 22 rollon the spar surface. The right-hand side of FIG. 2 show an alternativesituation wherein idler wheels 18 of the inverted tractor vehicle 12contact and roll on the external surface of bottom skin 6 while verticalidler wheels 20 of trailer vehicle 14 (and also idler wheels of trailervehicle 16 not visible in FIG. 2) contact and roll on the internalsurface of bottom skin 6, and the horizontal idler wheels 22 roll on thespar surface.

In accordance with the embodiment partly depicted in FIGS. 2 and 3, thetractor vehicle 12 comprises a frame 24. Four idler wheels 18 (only twoof which are visible in each of FIGS. 2 and 3) are rotatably mounted toframe 24 in a conventional manner. (Alternative embodiments may includemore idler wheels.) The idler wheels 18 may be made of plastic and havesmooth contact surfaces. Tractor vehicle motion is enabled by driving adrive wheel 26 (also rotatably mounted to frame 24) to rotate. Drivewheel 26 is coupled to a motor via a transmission (neither are shown inFIGS. 2 and 3). The drive wheel 26 is positioned on the frame 24 so thatit is in frictional contact with skin 4 or 6 when idler wheels 18 are incontact with the same skin. The drive wheel 26 can be made of syntheticrubber material. The surface of the drive wheel may have a treadpattern. In addition, the tractor vehicle 12 carries multiple permanentmagnets 28. Each permanent magnet 28 has North and South poles,respectively indicated by letters “N” and “S” in the drawings.

Still referring to FIGS. 2 and 3, each trailer vehicle 14, 16 iscomprised of a respective frame 34. For each trailer vehicle, twovertical idler wheels 20 (only one of which is visible in FIG. 3) andfour horizontal idler wheels 22 (only two of which are visible in FIG.3) are rotatably mounted to frame 34 in a conventional manner.(Alternative embodiments may include more idler wheels.) Each trailervehicle 14, 16 carries multiple vertically mounted permanent magnets 36,the North poles of which are magnetically coupled to the South poles ofconfronting permanent magnets 28 carried by the tractor vehicle 12. Inthe design shown in FIGS. 2 and 3, each trailer has two verticallymounted permanent magnets 36, but other designs may use differentconfigurations. The positions and pole orientations of the magnets mayhave other configurations as long as the N-S pairing and relativealignment of the magnets between the tractor and trailer are preserved.

As seen in FIG. 3, in addition to being magnetically coupled to thetractor vehicle 12, the trailer vehicles 14 and 16 are magneticallycoupled to each other using additional sets of permanent magnets 38 and42. As seen in FIG. 2, trailer vehicle 14 carries four horizontallymounted permanent magnets 38. Trailer vehicle 16 also carries fourhorizontally mounted permanent magnets 42 (only two of which are visiblein FIG. 3), the poles of which are respectively magnetically coupled toopposing poles of the permanent magnets 38 on trailer vehicle 14. Thismagnetic coupling produces an attraction force that holds idler wheels22 of trailer vehicles 14 and 16 in contact with opposing surfaces of anintervening spar web 8 (shown in FIG. 3).

FIGS. 2 and 3 show the basic principle of placing magnetically coupledvehicles on the exterior and in the interior of a hollow structurecomprising top and bottom skins connected by at least two spars. Thatprinciple can be applied when scanning the surface skins of a hollowstructure using an externally mounted NDI sensor unit. In accordancewith some embodiments, the NDI sensor unit may comprise a linear arrayof ultrasonic transducers which can be acoustically coupled to theexternal surface of the skin being inspected. For example, the inspectedregion may be covered with a continuous stream of water to acousticallycouple the ultrasonic transducers to a top or bottom skin. Magneticallycoupled systems are well suited for operation with water in theenvironment.

In accordance with some embodiments disclosed below, an external mobileplatform may comprise two drive tractor vehicles pivotably coupled tofront and/or rear payload platforms (e.g., a crossbar bridge), eachtractor vehicle being magnetically coupled to a respective pair ofpassive trailer vehicles disposed inside the hollow structure. Inaccordance with other embodiments disclosed below, an external mobileplatform may comprise a single drive tractor vehicle coupled to frontand/or rear payload platforms (e.g., trailer vehicles).

As the tractor vehicle is driven to travel along a desired path on theouter surface of the top or bottom skin, it pulls and/or pushes one ormore external payload platforms. Each externally mounted tractor vehicleis magnetically coupled to a respective pair of passive trailersdisposed inside the wing box on opposing sides of a spar. The magneticcoupling system described with reference to FIGS. 2 and 3 keeps theinverted vehicles in contact with the surface skin which they ride on.The internal passive trailers roll along the surfaces of the spar, whichallows the scanning system to take advantage of the internal structureof the wing box as a guide to track properly along the surface skin.

Each tractor vehicle can be provided with a capability to vary theamount of magnetic coupling force by physically moving its magnets up ordown using motors that are under computer control. This allows theapparatus to match the magnetic coupling force to the thickness of thepart being inspected. In this case, as the part thickness varies alongthe length of the part, the magnetic coupling force is dynamicallyadjusted under computer control to reflect this. An externally mountedpayload trailer vehicle may be provided with the same capability. Afeedback sensor is needed to provide information required by the controlcomputer to adjust the magnet separation distance as the skin thicknessvaries. One sensor option is a wheel rotation encoder rotatably mountedto the frame of one of the trailer vehicles to provide displacement froma specified starting point along the length of the wing box (or otherstructure being inspected). This position information, along withpredetermined data about the thickness of the skin (either from a CADmodel or measured directly), can be used to determine the amount ofdisplacement needed for the movable magnet units on the tractor or on anexternal payload platform. By knowing the locations of each of themagnetic coupling units relative to the sensor, the desired separationat each of the magnets can be determined. FIGS. 2 and 3 do not show themeans for automatically adapting to the variable thickness of theintervening skin or panel (i.e., top skin 4 or bottom skin 6) by raisingor lowering the magnets (which magnet motion is indicated bydouble-headed arrows in FIGS. 2 and 3) on the tractor vehicle as itmoves along the structure being inspected. Further details concerningthe trailer-tractor configuration depicted in FIGS. 2 and 3 and otherembodiments are disclosed in U.S. patent application Ser. No.13/313,267, the disclosure of which is incorporated by reference hereinin its entirety.

The basic concept of the tractor/trailer transporter system describedabove can be adapted to provide an alternative solution for NDI scanningof wing box surface skins that is compatible with the process forscanning of the wing box interior surfaces disclosed in U.S. patentapplication Ser. No. 13/534,014. The system consists of smallercomponents that can be setup in new locations without the need forconstruction of extensive infrastructure. The entire NDI skin scanningsystem could be shipped in cases. The only needed local infrastructurewould be water, air, power and support structure for the horizontalstabilizer during inspection.

The apparatus and methods disclosed herein enable maintenance toolscanning of surface skins using a magnetically coupled crawler vehicle.In cases where the maintenance tool is a sensor that needs to contactthe scanned surface, the scanning mechanism may comprise a sensorattachment mounted in such a way as to provide compliance between thesensor and the scanned surface. A further feature is amulti-configuration support tool that enables scanning of the bottomsurface skin of a wing box. The scanning method includes motion planningthat enables the collections of the scan strips on the top and bottomskins without turning the wing box over.

The skin scanning system comprises at least one drive tractor platform,e.g., a tractor vehicle, and at least one payload platform, e.g., atrailer vehicle coupled to a tractor vehicle or a crossbar bridgecoupled to a pair of tractor vehicles, that is pushed or pulled by thetractor. The tractor and payload platforms are coupled to each other,which coupling may be a mechanical or magnetic coupling. The tractorvehicle may comprise multiple motors, including a motor for driving amain drive wheel and motors for controlling the adaptive magneticcoupling system (which moves the coupling magnets in order to maintainrequired magnetic attraction force for variable surface thickness)onboard the tractor vehicle. The payload platform may comprise multiplemotors, including a motor for moving the payload (e.g., an NDI sensor)in a lateral direction, i.e., generally transverse to the direction ofmotion of the tractor, and in some embodiments, motors for controllingan adaptive magnetic coupling system onboard the payload platform. Thepayload platform may also have one or more rotation wheel encoders tomeasure distance traveled in the X direction due to motion generated bythe tractor drive motor(s), and the payload platform may have anotherrotational encoder to measure the distance that the sensor has moved inthe Y direction due to motion generated by the payload motion motor. Allof the motors carried by the external tractor and payload platforms arecomputer controlled. In contrast, the trailer vehicles inside the wingbox (e.g., a horizontal stabilizer) may be passive components. Theconnections to a computer from the vehicles are through a communicationcable that is controlled by a separate cable management device, thestructure of which is disclosed in U.S. patent application Ser. No.13/534,014.

FIG. 4 shows a top view of an external skin scanning apparatus atop asurface skin of a wing box 2, the apparatus having a double-tractortransporter configuration in accordance with one embodiment. Theapparatus is shown in two possible positions. The double-tractortransporter configuration for scanning a wing box skin 4 uses two drivetractor units 12 a, 12 b that are similar in design to the tractorpreviously described with reference to FIGS. 2 and 3. The tractorvehicles 12 a, 12 b ride on the exterior skin surface of the wing box.The tractors are magnetically coupled to respective pairs of passivetrailers (not visible in FIG. 4, but see FIG. 3) on the internal surfaceof skin 4 and spars 8 of the wing box 2. They are connected together bya payload platform 44 comprising a frame 50 that may ride on rollingelements, e.g., wheels (not visible in FIG. 4). Alternately, the framemay be attached by coupling elements directly to the tractor (such as acantilevered support arrangement). Frame 50 comprises a crossbar 66 thatbridges the two tractor vehicles 12 a, 12 b. The payload platform 44further comprises an NDI sensor array 40 (e.g., a linear array ofultrasonic transducers) attached to a drive nut 54. The drive nut 54 isthreadably coupled to a lead screw 52 that is rotatably coupled to frame50. The scan plane of NDI sensor array 40 is oriented perpendicular tothe axis of lead screw 52. The payload platform 44 further comprisesalignment guide elements 62 and 64 that guide the NDI sensor array 40along a linear path. The NDI sensor array 40 is housed in a shoe (notshown) that slides along the alignment guide elements 62, 64. Thepayload platform 44 further comprises a motor (e.g., a stepper motor)(not shown in FIG. 4) which drives rotation of the lead screw 52. Inresponse to lead screw rotation, the NDI sensor array 40 will translatealong the length of frame 50, i.e., in a lateral direction relative tothe spar 8 underlying the tractor vehicle 12 a. The NDI sensor array 40is preferably mounted via a mechanism (e.g., a spring-loaded shoe) thatprovides a sufficient amount of vertical compliance to enable the arrayto stay in contact with a curved aerodynamic surface of a horizontalstabilizer.

The payload platform 44 and the tractor vehicles 12 a, 12 b are coupledtogether to form a chassis that is movable in a spanwise direction alonga wing box for an airplane. For scanning a surface skin of a horizontalstabilizer, the tractor vehicles 12 a, 12 b cannot be rigidly coupled bythe crossbar 66 since they have to stay on the surface of skin 4 andthat surface is not flat. In addition, the tractor motions will not beparallel, since the spars 8 inside the horizontal stabilizer are notparallel. In the setup shown in FIG. 4, the tractor-platform couplingsallow pivoting between the two tractors, as well as extension andcontraction. Another option is to have the tractors ride on one path onthe crossbar and the sensor array ride on another. The system has onetractor to be coupled to the crossbar 66 with a sliding and rotatingjoint, while the other tractor is coupled by a rotating joint. In theembodiment shown in FIG. 4, the tractor vehicle 12 a (i.e., the firstchassis part) is pivotably coupled to the payload platform 44 (i.e., thesecond chassis part) by a hitch type of coupling mechanism (e.g., a balljoint) (not visible in FIG. 4). In addition, the tractor vehicle 12 b(i.e., the third chassis part) is pivotably coupled and slidably coupledto the payload platform 44. The mechanism for coupling tractor vehicle12 b to payload platform 44 comprises a ball joint (not visible in FIG.4) connected to a pin 60 which slides freely in a slot 58 formed incrossbar 66 of frame 50.

In accordance with an alternative embodiment in which the frame has acantilevered configuration, a joint with only two degrees of freedom(such as two revolute joints) can be employed instead of a ball joint.

The magnetic couplings between the external tractor vehicles and theinternal passive trailer vehicles, with respective skin-spar jointstherebetween, couples the trailer vehicles to the wing box. Since thepayload platform is coupled to and its motion is constrained by thetrailer vehicles, the coupling of the trailer vehicles to respectivespars has the effect of coupling the payload platform to the wing box.Thus the payload platform does not require means for gripping parts(e.g., the leading and trailing edges) of the wing box.

For the usage setup shown in FIG. 4, the two tractors 12 a, 12 b and thepayload platform 44 can be positioned on one end of the horizontalstabilizer, and the process is started by moving the NDI sensor array 40in the Y direction all the way from one edge of the wing box to theother edge. After the array has been stopped, both tractors are movedsimultaneously a few inches ahead. Then the Y direction scan isrepeated, except that the array is now moved in the opposite direction.Again the array is stopped and then the tractors are moved another fewinches ahead. This scanning process can be repeated until the entiresurface skin 4 has been scanned. The path trace for this scanning modeis shown in FIG. 5.

In accordance with a variation of the embodiment shown in FIG. 4, theNDI sensor array 40 can be rotated 90 degrees (i.e., oriented parallelto the axis of lead screw 52), as shown in FIG. 6. A frame 51 (which canbe narrower than frame 50 seen in FIG. 4 due to the change in arrayorientation) has a crossbar 66 that bridges two tractor vehicles 12 a,12 b. In this case the scanning process is started by indexing the NDIsensor array 40 to the starting position along the frame 51, i.e., inthe Y direction. Then the tractors 12 a, 12 b are moved all the wayalong the length of respective spars 8 for each Y direction indexposition. The path trace for a potential path plan for this scanningmode is shown in FIG. 7.

In the examples shown in FIGS. 4 and 6, when the NDI sensor array 40cannot overlie the marginal strip adjacent one end of wing box 2 becausethe transporter (i.e., tractor vehicle and magnetically coupled internaltrailer vehicles) has reached that end and can travel no further, arun-on/run-off component can be attached to the end of the wing box,which component is configured to enable the transporter to travel beyond(i.e., run-off) that end of the wing box. The same run-on/run-offcomponent allows scanning to start at the same end of the wing box, inwhich case the transporter mechanisms runs onto the wing box from therun-on/run-off component. A pair of run-on/run-off components (i.e.,each component can serve either function depending on whether thetrailer vehicle is leading or trailing the payload platform) can be usedto allow the externally mounted tractor vehicle and the internallymounted passive trailer vehicles to start partially off of or runpartially off of either end of the wing box. These components will berespectively attached to the start and end positions and will allow themore centrally positioned NDI sensor array to cover the entire length ofthe wing box skin. These run-on/run-off components (which may be made ofplastic or composite material) are sized and shaped to match theparticular wing box being inspected and are different for the root andtip ends of the wing box. They may be clamped or taped in place.

FIG. 8 is a top view of the external skin scanning apparatus depicted inFIG. 6, wherein the tractor vehicles 12 a and 12 b are supported by arun-on/run-off component 80 that has been attached to the root end of awing box. In this implementation, run-on/run-off component 80 comprisesa top skin 81, a bottom skin (not visible in FIG. 8), and a multiplicityof spars 82 which respectively align with the spars and skins of thewing box. This enables each tractor vehicle 12 a, 12 b and itsassociated pair of internal passive trailer vehicles (not visible inFIG. 8) to transition smoothly from the wing box to the run-on/run-offcomponent 80 (or vice versa) while maintaining their magnetic couplingto each other.

FIG. 9 shows a top view of an external skin scanning apparatus inaccordance with a variation of the embodiment shown in FIG. 4. Thisapparatus has front and rear payload platforms 44 a and 44 b coupled tofront and read ends respectively of two tractor vehicles 12 a, 12 b. Inthe scenario depicted in FIG. 9, the tractor vehicles are traveling inthe direction indicated by the arrows, in which case the tractors arepushing payload platform 44 a and pulling payload platform 44 b. Eachpayload platform 44 a, 44 b shown in FIG. 9 has a construction similarto that of payload platform 44 previously described with reference toFIG. 4, which description shall not be repeated here for the sake ofbrevity. One difference is that the frame 50 for payload platform 44 acan be shorter in length than frame 50 of payload platform 44 b due tothe narrowing of the width of the skin as the apparatus moves toward thetip end of the wing box. Another difference is that the slots 60 of therespective frames 50 of payload platforms 44 a, 44 b are offset due tothe fact that the spars 8 are converging, not parallel, as they extendfrom the root end to the tip end.

The apparatus shown in FIG. 9 is equipped with front and rear NDI sensorunits 40 which can be operated independently. When payload platform 44 boverlies the lateral marginal strip of skin surface adjacent the rootend of the wing box, its NDI sensor unit 40 can inspect that stripwithout using a run-on/run-off component. Likewise when payload platform44 a overlies the lateral marginal strip of skin surface adjacent thetip end of the wing box, its NDI sensor unit 40 can inspect that stripwithout using a run-on/run-off component. The front and rear scanningunits can be operated with synchronized motions in the path plan thatinvolves moving the tractor to the appropriate positions to avoidoverlap of the front and rear scans. This type of path plan may includescan groups that are the length of the spacing between the front andrear platforms, or it may include interlacing of the front and rear scanstrips.

FIG. 10 shows a top view of an external skin scanning apparatus atop asurface skin of a wing box 2, the apparatus having a single-tractortransporter configuration in accordance with a further embodiment. Theapparatus is shown in two possible positions. The single-tractortransporter configuration for scanning a wing box skin 4 comprises adrive tractor unit 12, similar in design to the tractor previouslydescribed with reference to FIGS. 2 and 3, coupled to an NDI scannertrailer vehicle 68.

The NDI scanner trailer vehicle 68 comprises an NDI sensor array 40(e.g., a linear array of ultrasonic transducers) carried by a frame. Theframe comprises first and second rolling frame parts 70 a, 70 b rigidlyconnected by a central frame part 72. A rotation encoder 46 may bemounted to the frame of NDI scanner trailer vehicle 68. In theimplementation shown in FIG. 10, the rotation encoder 46 is mounted tothe first rolling frame part 70 a. An encoder wheel 48 is coupled to therotation encoder 46. The rotation encoder outputs a respective pulse foreach incremental angular rotation by encoder wheel 48 during travel ofNDI scanner trailer vehicle 68 along a spar, providing an indication ofthe X position of the NDI sensor array 40. Similarly, a lead screwencoder (not shown in FIG. 10) can provide pulses indicating the Yposition of the NDI sensor array 40 in a well-known manner.

The NDI sensor array 40 is slidably coupled to the central frame part72, the latter comprising alignment guide elements (as previouslydescribed) that guide the NDI sensor array 40 along a linear path. TheNDI sensor array 40 is coupled to a drive nut 54, which is threadablycoupled to a lead screw 52 that is rotatably coupled to central framepart 72. In the example depicted in FIG. 10, the scan plane of the NDIsensor array 40 is oriented parallel to the axis of lead screw 52.Alternatively, the scan plane could be perpendicular to the lead screw(as seen in FIG. 4). The NDI scanner trailer vehicle 68 furthercomprises a motor (e.g., a stepper motor) (not shown in FIG. 10) whichdrives rotation of the lead screw 52. In response to lead screwrotation, the NDI sensor array 40 will translate along the length ofcentral frame part 72, i.e., in a lateral direction relative to the sparunderlying the tractor vehicle 12. The NDI sensor array 40 is preferablymounted via a mechanism (e.g., a spring-loaded shoe) that provides asufficient amount of vertical compliance to enable the array to stay incontact with a curved aerodynamic surface of a horizontal stabilizer.

As shown in FIGS. 10A and 10B, the tractor vehicle 12 and NDI scannertrailer vehicle 68 can ride on either the exterior surface of top skin 4or the exterior surface of bottom skin 6. In the latter case, thetractor vehicle 12 and NDI scanner trailer vehicle 68 will be inverted.The tractor vehicle 12 and NDI scanner trailer vehicle 68 aremagnetically coupled to respective pairs of internal passive trailervehicles. Each pair of internal passive trailer vehicles ride on theinterior surface of the intervening skin and on opposing sides of a spar8. Two internal passive trailer vehicles 74 a and 76 b are visible inFIG. 10A; the other two internal passive trailer vehicles 74 b and 76 bare visible in FIG. 10B. The internal passive trailer vehicles 74 a,bare magnetically coupled to each other through a spar 8, as are internalpassive trailer vehicles 76 a,b.

The tractor-trailer combination shown in FIG. 10 can be placed atdiscrete positions along the width of the wing box associated with thelocations of the respective interior spars. In the scenario depicted onthe left-hand side of FIG. 10, the tractor vehicle 12 pushes the NDIscanner trailer vehicle 68 along a path dictated by spar 8 a, whichguides the internal passive trailer vehicles. Similarly, in the scenariodepicted on the right-hand side of FIG. 10, the tractor vehicle 12 pullsthe NDI scanner trailer vehicle 68 along a path dictated by spar 8 b,which guides the internal passive trailer vehicles.

The usage setup for the configuration shown in FIG. 10 would be toposition the tractor 12 and a portion of the NDI scanner trailer 68 on arun-on/run-off component (not shown in FIG. 10) attached to one end ofthe horizontal stabilizer, and start the scanning process by firstmoving the NDI sensor array to the starting position (e.g., the leftside in this example), and then moving the tractor onto the wing box andthen along the length of the wing box until the NDI sensor array reachesthe other end of the wing box. After the tractor has been stopped, theNDI sensor array 40 is moved in the Y direction along the length of thecentral frame part 72 to its next index position (to the right in thisexample). After the array has been stopped, the tractor is moved in theopposite direction along the length of the wing box until the trailer isagain situated on the run-on/run-off component. After the tractor hasbeen stopped on the run-on/run-off component, the NDI sensor array ismoved in the Y direction to its next index position. Again the array isstopped and the foregoing steps are repeated until the scan with the NDIsensor array in its final index position (the right-most position inthis example) is completed. All of the components are then removed fromspar 8 a and moved to the next spar to be scanned, and the process isrepeated. The path trace for this scanning mode when the apparatus istraveling along internal spar 8 a is shown in FIG. 11. Path tracessimilar to the one shown in FIG. 11 will be generated for each internalspar of the wing box when the entire top skin 4 is scanned.

In accordance with an alternative embodiment having a single tractorconfiguration, the scan plane of the NDI sensor array can be orientedperpendicular to the lead screw. The path trace for this scanning modewhen the apparatus is traveling along internal spar 8 a is shown in FIG.12.

Using a single tractor would have several advantages over thedouble-tractor setup, but would require that the system scan arespective section of the horizontal stabilizer skin (approximately 2feet wide) above each spar, and then move the tractor-trailer setup overto the next spar and repeat the process. For the horizontal stabilizerapplication described here, the single-tractor configuration wouldresult in five sets of long NDI scan strips instead of one large scan.Accordingly, if something goes wrong with the large scan produced by thedouble-tractor configuration, the system operator may need to run theentire process again, but with the single-tractor configuration, thesystem operator would only need to re-run one of the five scan plans.

FIG. 13 shows a top view of an external skin scanning apparatus inaccordance with a further embodiment. This apparatus has front and rearpayload platforms 90 a and 90 b coupled to front and read ends of atractor vehicle 12. In the scenario depicted in FIG. 13, the tractorvehicle 12 is traveling in the direction indicated by the arrow, inwhich case the tractor is pushing payload platform 90 a and pullingpayload platform 90 b. Each payload platform 44 a, 44 b shown in FIG. 13has a construction similar to that of payload platform 44 previouslydescribed with reference to FIG. 4, but is shorter in length.

The apparatus shown in FIG. 13 is equipped with front and rear NDIsensor units 40 which can be operated independently. When payloadplatform 90 b overlies the lateral marginal strip of skin surfaceadjacent the root end of the wing box, its NDI sensor unit 40 caninspect that strip without using a run-on/run-off component. Likewisewhen payload platform 90 a overlies the lateral marginal strip of skinsurface adjacent the tip end of the wing box, its NDI sensor unit 40 caninspect that strip without using a run-on/run-off component.

An extension of the single tractor/single NDI scanner trailer setupshown in FIG. 10 is to have respective NDI scanner trailers connected tothe front and rear of the tractor. This would allow faster scanning, andthe NDI sensor array could reach the edges of both the root and tip endsof the horizontal stabilizer without changing the vehicle configuration.This option is shown in FIGS. 14, 14A and 14B.

FIG. 14 shows a top view of an external skin scanning apparatuscomprising a single tractor 12 coupled to front and rear NDI scannertrailers 92, 94 in accordance with a further embodiment. Each NDIscanner trailer vehicle comprises an NDI sensor array 40 slidablycoupled to a respective frame. The frame of NDI scanner trailer 92comprises first and second rolling frame parts 96 a, 96 b rigidlyconnected by a central frame part 100; the frame of NDI scanner trailer94 comprises first and second rolling frame parts 98 a, 98 b rigidlyconnected by a central frame part 102. The coupled frames of tractorvehicle 12 and trailer vehicles 92, 94 form respective parts of achassis, the three coupled chassis parts forming a mobile platform(consistent with terminology used in the appended claims). A rotationencoder 46 may be mounted to the frame of either NDI scanner trailervehicle. In the implementation shown in FIG. 14, the rotation encoder 46is mounted to the first rolling frame part 96 a. An encoder wheel 48 iscoupled to a rotation encoder 46 which outputs pulses for tracking the Xpositions of both NDI sensor arrays 40 (which are separated by a knowndistance). Optionally, a second rotation encoder/encoder wheel assemblycould also be mounted to trailer vehicle 94. In addition, respectivelead screw encoders (not shown in FIG. 14) can provide pulses indicatingthe Y positions of the NDI sensor arrays 40.

Each NDI sensor array 40 seen in FIG. 14 is slidably coupled to arespective central frame part 100, 102 having alignment guide elements(as previously described) that guide the NDI sensor array 40 along alinear path. Each NDI sensor array 40 is coupled to a respective drivenut 54. In the example depicted in FIG. 14, the scan planes of the NDIsensor arrays 40 are oriented perpendicular to the axis of lead screw52. Alternatively, the scan planes could be parallel to the lead screw.Each NDI scanner trailer vehicle 92, 94 further comprises a motor (notshown) which drives rotation of the lead screw 52 and motors (not shown)for raising or lowering the magnets 28 which couple the NDI scannertrailer vehicles to internal passive trailer vehicles. In accordancewith one implementation, the motor which drives rotation of the leadscrew 52 can be mounted to the central frame part, while two motors forchanging the elevation of respective groups of magnets (four magnets ineach group) are respectively mounted to the first and second rollingframe parts.

In this embodiment, the scan plane of each NDI sensor unit 40 isoriented perpendicular to the lead screw 52 to which it is rotatablycoupled (by means of a respective drive nut 54). Each NDI sensor unit 40is movable in a direction perpendicular to the direction of tractortravel (indicated by an arrow in FIG. 14).

FIG. 14A shows a side view of a configuration in which a scanningapparatus of the type shown in FIG. 14 is magnetically coupled torespective sets of passive trailer vehicles disposed on opposing sidesof a spar 8 and magnetically coupled to each other. FIG. 14A shows aninspection scenario wherein the scanning apparatus is atop the top skin4. As shown in FIG. 14A, the tractor vehicle 12 and NDI scanner trailervehicles 92, 94 can ride on the exterior surface of top skin 4. Thetractor vehicle 12 and NDI scanner trailer vehicles 92, 94 aremagnetically coupled to respective pairs of internal passive trailervehicles. Each pair of internal passive trailer vehicles ride on theinterior surface of the intervening skin and on opposing sides of spar8. Three internal passive trailer vehicles 74, 76 and 78 are visible inFIG. 14A, within another set of three internal passive trailer vehiclesbeing disposed behind spar 8. Thus the mobile platform shown in FIG. 14Aincludes nine vehicles which move in unison. The tractor vehicle 12 iscoupled to NDI scanner trailer vehicles 92, 94, a first pair of internalpassive trailer vehicles is coupled to tractor vehicle 12 and eachother, a second pair of internal passive trailer vehicles is coupled toNDI scanner trailer vehicle 92 and each other, and a third pair ofinternal passive trailer vehicles is coupled to NDI scanner trailervehicle 94 and each other.

FIG. 14B shows an end view of the apparatus depicted in FIG. 14A.Internal passive trailer vehicle 78, which is magnetically coupled toNDI scanner trailer vehicle 94 through skin 4, is also magneticallycoupled to internal passive trailer vehicle 104 through spar 8, which isalso magnetically coupled to NDI scanner trailer vehicle 94 through skin4. The other internal passive trailer vehicles are not visible in theend view of FIG. 14B.

In this single-tractor design, a linear translational element (e.g., alead screw) is used for moving the NDI sensor array laterally. However,there are other mechanisms that could be used for lateral positioning.For example, a linkage device or a multiple degree-of-freedomarticulated arm can be used. These types of designs would lower theoverall width of the system, which could be useful in passing throughthe horizontal stabilizer support tool, which will be discussed next.

Part Holding Tools

One feature of the skin scanning apparatus disclosed herein is itsability to run on the bottom as well as the top of the part beinginspected. To run on the bottom, the externally mounted platform, i.e.,the tractor vehicle and tool-carrying chassis parts, needs a clear paththat avoids any structural supports. As part of this scanning system,part holding tools were designed that support the part at opposite endsthereof.

FIG. 15A shows side view of configurable tools that can be used tosupport a wing box 2 during non-destructive inspection using systemsdisclosed herein. One part holding tool (hereinafter “inboard supporttool”) 120 is designed to hold/support a wing box 2 near its root end;another part holding tool (hereinafter “outboard support tool”) 126 isdesigned to hold/support wing box 2 near its tip end. These inboard andoutboard part holding tools may be constructed differently to reflectthe different sizes, shapes and weights of the root and tip ends of thewing box 2. As seen in FIG. 15A, a third part holding tool (hereinafter“alternate inboard support tool”) 122 is designed to hold/support wingbox 2 at an intermediate position closer to the position of inboardsupport tool 120 than to the position of outboard support tool 126,while a fourth part holding tool (hereinafter “alternate outboardsupport tool”) 124 is designed to hold/support wing box 2 at anintermediate position closer to the position of outboard support tool126 than to the position of inboard support tool 120.

Each support tool 120, 122, 124 and 126 comprises a respective pedestal128 that stands on the ground and a frame 130 supported by the pedestal128. Each support tool has movable support structure, i.e., a row ofheaders 132, arranged in a chordwise direction beneath the wing, whichrow of headers can be raised to provide support for the wing box 2 orlowered to provide a clear channel for passage of the externally mountedmobile platform during inspection of the bottom skin 6 of the wing box2. Each row of headers is attached to and vertically displaceable bypistons of a respective pair of pneumatic cylinders (not shown) situatedon opposite sides of a respective frame 130. Each row of headers can bemoved up and down independently. Each pneumatic cylinder can beselectively supplied with pressurized air from a source via an airdistribution system (not shown). In one implementation, the pneumaticcylinders are actuated by manual operation of header controls. In otherembodiments, the air flow to the pneumatic cylinders can be automatedand be included as an instruction in the motion path plan.

In order for the skin scanning apparatus shown in FIG. 4 to be able toscan the full range of both the top and bottom aerodynamic surfaces, thewing box support system shown in FIG. 15A can be configured to allow theexternal mobile platform (which carries one or more NDI sensor arrays)to get past the headers of the support tools.

For some single-tractor configurations, it may be sufficient to simplyprovide pass-through openings of sufficient width between the extendedheaders of inboard and outboard support tools 120 and 126 while theheaders of alternate inboard and outboard support tools 122 and 124remain in a retracted state. But for double-tractor configurations (suchas the apparatus shown in FIG. 4), all of the headers of any one of thesupport tools would need to be retracted in order to allow theexternally mounted mobile platform to pass between that support tool andthe bottom skin 6 of the wing box 2.

FIGS. 15A through 15C show three configurations which may occur during aconfiguration change sequence that enables the bottom skin 6 to beinspected along its entire length while maintaining vertical support ofthe wing box 2. In the first stage of the configuration change process(shown in FIG. 15A), an externally mounted NDI scanner platform 140starts to the left of the inboard support tool 120 while the headers 132of inboard and outboard support tools 120 and 126 are in their extendedpositions to support the wing box 2 and the headers 132 of alternateinboard and outboard support tools 122 and 124 are retracted.

In the second stage (shown in FIG. 15B), the headers of the alternateinboard support tool 122 are extended (i.e., raised) to support the wingbox 2 and the headers of the inboard support tool 120 are retracted(i.e., lowered) so that the wing box is now supported by alternateinboard support tool 122 and outboard support tool 126. In thisconfiguration, the NDI scanner platform 140 can be moved through thespace previously obstructed by the extended headers 132 of inboardsupport tool 120 to a position whereat the NDI scanner array overlies aspace located between the inboard support tool 120 and the alternateinboard support tool 122.

In the third stage (shown in FIG. 15C), the headers of the inboardsupport tool 120 are extended to support the wing box 2 and the headersof the alternate inboard support tool 122 are retracted so that the wingbox is now again supported by the inboard and outboard support tools 120and 126. In this configuration, the NDI scanner platform 140 can bemoved through the space previously obstructed by the extended headers ofalternate inboard support tool 122 to a position whereat the NDI scannerarray overlies a space located between the alternated inboard andoutboard support tools 122 and 124.

A similar process happens when the mobile platform 140 reaches theoutboard support tooling. Note that the vertical support transition onlyneeds to take place when the wide section of the mobile platform passesthrough the support region, since the tractor vehicle already fits inand can be passed through the space between adjacent headers in thesupport tool.

Other System Use Cases

Up to this point, only initial inspection during manufacturing has beendiscussed, in which case the number of obstructions on the surfaces isminimized (since other components have not been attached yet), but useof this system for in-service inspection is also a possibility. Oneproblem with in-service inspection would be avoiding fasteners throughthe flanges of the wing box. This is mainly a concern for themagnetically coupled followers. Some interfering parts on the objectbeing scanned may be removed during inspection, but for those thatcannot be removed, one option here is to modify the shape of themagnetically coupled followers to have gaps or cutouts to allow the tofasteners to pass under the followers.

One existing solution for wing box inspection uses a gantry system toposition the NDI sensor array. That system is large and expensive, isinstalled in a fixed position, and takes additional training to operate.The apparatus disclosed herein is less expensive, smaller, portable, andis compatible with the system disclosed in U.S. patent application Ser.No. 13/534,014, which would require less additional training foroperators. The gantry-based solution requires that the horizontalstabilizer be turned over in order to scan the other aerodynamicsurface. This step would not be needed using the process disclosedherein.

Computer System and Software

Regardless of which configuration is used, the active system componentscan be controlled by a computer system in response to commands input viaa graphical user interface by the system operator or through anautomated process using pre-planned motion instructions to control thesystem. The motors onboard the trailer vehicles and NDI scanner trailervehicles or platforms are connected to an electronic control box bymeans of flexible electrical cables. The electronic control box containsthe system power supplies and integrates all the scanner controlconnections and provides an interface between the computer and thescanners and tractor.

FIG. 16 is a block diagram showing some components of a control systemin accordance with the embodiment depicted in FIG. 10. The controlsystem comprises a ground-based computer 182 programmed with motioncontrol application software 198 and NDI scan application software 200.The two-way communication between these software modules (indicated by atwo-headed arrow in FIG. 16) is optional. In the alternative, the motioncontrol application software 198 and NDI scan application software 200could be run on separate computers.

In the embodiment shown in FIG. 16, the control computer 182 isconnected to a drive tractor platform in the form of a tractor vehicle12 and to a payload platform 44 by flexible electrical cables thatconnect to an electronics relay/switch box 180. The electronicrelay/switch box 180 contains the system power supplies, relays, and adata acquisition device 190, and integrates all the scanner controlconnections and provides an interface between the computer 182 and thetractor vehicle 12, the payload platform 44, and a cable managementsystem 183

The computer 182 may comprise a general-purpose computer programmed withmotion control application software 198 comprising respective softwaremodules for controlling drive motor 186 and magnet vertical positioningmotors 188 onboard the drive tractor platform 12, lead-screw motor 194onboard the payload platform 44, and cable motor 184 of the cablemanagement system 183. Each magnet motor 188 displaces a group oftractor coupling magnets 28 as disclosed in U.S. patent application Ser.No. 13/313,267. Motion control application software 198 controls thelead screw motor 194 to drive rotation of the lead screw 196, whichrotation causes the ultrasonic transducer array 150 to scan in a Ydirection. The lead screw 196 is connected to an output shaft of thelead screw motor 194. The range of motion of the ultrasonic transducerarray 150 in both Y directions is limited by limit switches 192. AY-axis encoder 154 measures the angular position of the output shaft oflead screw motor 194, which angular position is proportional to the Yposition of the ultrasonic transducer array 150. The motion controlapplication software 198 is capable of moving array 150 in the Ydirection and tractor 12 in the X direction independently andalternatingly. The X and Y positions of the ultrasonic transducer array150 are respectively measured by pulses output from the X- and Y-axisencoders 152, 154.

In accordance with one embodiment, the encoded data from both encoders152 and 154 is received by a data acquisition device 190 via a relayswitch and a splitter (not shown) inside the electronics box 180. Thedata acquisition device 190 also has digital input and outputconnections that are used for multiple functions within the system. Inaccordance with other embodiments in which alternate forms of the NDIsensor actuator mechanism (such as a linkage-based mechanism) do notproduce linear output motion, the data acquisition device 190 may beused to generate quadrature pulses that simulate the encoder pulseswhich would be outputted if a position encoder were arranged to outputpulses representing the Y position of ultrasonic transducer array 150.These simulated encoder pulses are sent to the ultrasonicpulser/receiver 202. The ultrasonic pulser/receiver also receives pulsesgenerated by the X-axis encoder 152 via the aforementioned switch andsplitter (not shown in FIG. 16) inside the electronic box 180. Thepulser/receiver 202 sends the encoder pulses to the NDI scan software200. The NDI scanning software application 200 interprets the simulatedencoder pulses as Y-encoder values, which are used (along with theX-encoder values) to position the scan data from array 150 in the properlocations.

The computer 182 may also host NDI scan acquisition and display software200 that controls the ultrasonic pulser/receiver 202. The ultrasonicpulser/receiver 202 in turn sends pulses to and receives return signalsfrom the ultrasonic transducer array 150. The NDI scan applicationsoftware 200 controls all details of the scan data and the display ofdata. In the embodiment shown in FIG. 16 the pulser/receiver 202correlates the acquired ultrasonic data with the encoder data receivedfrom the data acquisition device 190.

The motion control application software 198 also controls a motor 184 ofthe cable management system 183. There are several cables that need toaccompany the scanner and the tractor down the length of the box beinginspected. The cable management system automatically feeds out thecables or pulls in the slack as the vehicles move. The cable managementsystem 183 consists of two sets of motorized wheels (not shown) thatgrip the cables. The cable motor 184 is under computer control by way ofcontrol PC 182 and motion control software 198, which synchronizes thecables with the movement of the tractor, extending or retracting thecables as appropriate.

Software on the computer that controls the movement of the motors on thetractor, trailer, and cable management device allows scriptable motionplans to be created and executed to produce automated motion control ofthe system. Operators can load and activate custom motion path plansusing a graphical use interface 178, which shows the status of thescanning process displayed on a virtual representation of the horizontalstabilizer. The graphical use interface 178 may be of the type disclosedin U.S. patent application Ser. No. 13/534,014. In addition toinitiation of automated motion, the graphical user interface 178 alsoallows the operator to issue direct motion commands.

FIG. 17 is a diagram representing a screen shot 200 of a graphical userinterface in accordance with one embodiment. Each graphical userinterface display dialog (not shown) displays a two-dimensional visualrepresentation 202 of the target object to be scanned, which in thisexample is a horizontal stabilizer. In this visual representation, thehorizontal bands 204 and 206 respectively represent top and bottom skinsof the horizontal stabilizer, while vertical bands 208 represent thespars which connect the top and bottom skins. Layered over thetwo-dimensional target object representation are a series of virtualbuttons 210 (hereinafter “buttons”) that represent the individual motionpaths that can be selected and executed. Internally these buttons areassociated with specific motion script files stored in the control PCthat contain the parameters associated with that specific path. Thebuttons 210, which can be used to select the active motion path, arepositioned in a way that they correspond to the actual position of thescanning devices on the part being scanned from the operator's point ofview. This one-to-one correspondence makes it easier to keep track ofwhich motion path sequence will be used, as well as marking which scanshave been completed. The current motion path is indicated by a shadedbutton 212; empty buttons 210 indicate which scans have not beencompleted yet. This symbology helps the operator keep track of thecurrent scan path, the areas that have been scanned, and the areas thatstill need to be scanned. This graphical user interface gives a simplevisual representation that is easy to use and can be operated withlittle additional training.

In accordance with the embodiments described above, a control computeris provided with encoder information concerning the spanwise andchordwise positions of the inspection chassis relative to the frame ofreference of the wing box being inspected. In the alternative, thisfunctionality can be provided by any one of a multiplicity of knownpositional tracking mechanisms. In accordance with various alternativeembodiments, an optical tracking system can be used to determine thespanwise position of the inspection. For example, U.S. Pat. No.7,643,893 discloses a motion capture system wherein multiple motioncapture cameras are set up around the object to be scanned to create athree-dimensional capture volume that captures motion for all sixdegrees-of-freedom of the object being tracked. Alternatively, theoptical tracking mechanism may comprise a local positioning system ofthe type disclosed in U.S. Pat. No. 7,859,655.

In addition to NDI-specific types of inspection, other types ofinspection or manufacturing applications may be able to take advantageof the mechanical and control concepts presented here. For example, theNDI sensor carried by the payload platform can be replaced by othercomponents, such as: laser scanners, video cameras, roboticmanipulators, reflective targets, paint heads, or otherelectro-mechanical components.

While skin scanning systems have been described with reference tovarious embodiments, it will be understood by those skilled in the artthat various changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the teachingsherein. In addition, many modifications may be made to adapt theteachings herein to a particular situation without departing from thescope thereof. Therefore it is intended that the claims not be limitedto the particular embodiments disclosed herein.

As used in the claims, the term “computer system” should be construedbroadly to encompass a system having at least one computer or processor,and which may have multiple computers or processors that communicatethrough a network or bus. As used in the preceding sentence, the terms“computer” and “processor” both refer to devices comprising a processingunit (e.g., a central processing unit) and some form of memory (i.e.,computer-readable medium) for storing a program which is readable by theprocessing unit.

The method claims set forth hereinafter should not be construed torequire that the steps recited therein be performed in alphabeticalorder (alphabetical ordering in the claims is used solely for thepurpose of referencing previously recited steps) or in the order inwhich they are recited. Nor should they be construed to exclude anyportions of two or more steps being performed concurrently oralternatingly.

1. A method for moving a maintenance tool over a surface of a hollowstructure having a skin connected to at least one vertical structuralelement, the method comprising: (a) placing a first tractor vehicle in aposition external to the hollow structure and in contact with the skin;(b) placing first and second trailer vehicles in respective interiorspaces of the hollow structure with a first vertical structural elementof the hollow structure therebetween; (c) magnetically coupling thefirst and second trailer vehicles to the first tractor vehicle with theskin therebetween and to each other with the first vertical structuralelement therebetween; (d) coupling a payload platform to the firsttractor vehicle in a position external to the hollow structure, thepayload platform comprising a frame and a maintenance tool that ismovable relative to the frame; (e) moving the first tractor vehiclealong a path that follows the first vertical structural element; (f)stopping the first tractor vehicle; and (g) moving the maintenance toolof the payload platform in a first direction relative to the frame ofthe payload platform while the first tractor vehicle is stopped in step(f).
 2. The method as recited in claim 1, wherein during step (g) themaintenance tool of the payload platform moves laterally relative to thefirst vertical structural element.
 3. The method as recited in claim 1,further comprising: (h) placing a second tractor vehicle in a positionexternal to the hollow structure and in contact with the skin; (i)placing third and fourth trailer vehicles in respective interior spacesof the hollow structure with a second vertical structural element of thehollow structure therebetween; (j) magnetically coupling the third andfourth trailer vehicles to the second tractor vehicle with the skintherebetween and to each other with the second vertical structuralelement therebetween; (k) coupling the payload platform to the secondtractor vehicle; (l) during step (e), moving the second tractor vehiclealong a path that follows the second vertical structural element; and(m) stopping the second tractor vehicle, wherein step (g) is performedwhile the first and second tractor vehicles are not moving.
 4. Themethod as recited in claim 3, wherein the first and second verticalstructural elements are not parallel.
 5. The method as recited in claim1, further comprising: (h) stopping the maintenance tool; (i) moving thefirst tractor vehicle further along the path that follows the firstvertical structural element; (j) stopping the first tractor vehicle; and(k) after step (j), moving the maintenance tool of the first payloadplatform in a second direction relative to the frame of the payloadplatform while the first tractor vehicle is stopped in step (j), thesecond direction being opposite to the first direction.
 6. The method asrecited in claim 1, further comprising actuating the maintenance tool toperform a maintenance function.
 7. The method as recited in claim 1,further comprising: placing first, second and third hollow structuresupport tools under the hollow structure, the first hollow structuresupport tool being closer to a root end of the hollow structure than isthe second hollow structure support tool and the third hollow structuresupport tool being closer to a tip end of the hollow structure than isthe second hollow structure support tool, each of the first, second andthird hollow structure support tools being configurable between a firststate wherein it supports the hollow structure and obstructs the payloadplatform and a second state wherein it neither supports the hollowstructure nor obstructs the payload platform; configuring the first,second and third hollow structure support tools so that the second andthird hollow structure support tools support the hollow structure whilethe first hollow structure support tools does not; while the second andthird hollow structure support tools are supporting the hollowstructure, moving the first tractor vehicle from a position whereat thepayload platform overlies a space between the root end of the hollowstructure and the first hollow structure support tool to a positionwhereat the payload platform overlies a space between the first andsecond hollow structure support tools; after the preceding step has beenperformed, reconfiguring the first and second hollow structure supporttools so that the first and third hollow structure support tools supportthe hollow structure while the second hollow structure support toolsdoes not; and while the first and third hollow structure support toolsare supporting the hollow structure, moving the first tractor vehiclefrom the position whereat the payload platform overlies a space betweenthe first and second hollow structure support tools to a positionwhereat the payload platform overlies a space between the second andthird hollow structure support tools.
 8. An apparatus for performing amaintenance function on a skin of a hollow structure, the apparatuscomprising a first tractor trailer, first and second trailer vehicles, afirst payload platform coupled to the first trailer vehicle, and a firstmaintenance tool supported by and movable relative to and along a lengthof the first payload platform, wherein the first and second trailervehicles are magnetically coupled to the first tractor trailer and toeach other, and the first maintenance tool is one of the following: anon-destructive inspection unit, a drill, a scarfer, a grinder, afastener, an appliqué applicator, a ply mapper, a depainting tool, acleaning tool, and a painting tool.
 9. The apparatus as recited in claim8, further comprising a second tractor trailer, third and fourth trailervehicles, wherein the third and fourth trailer vehicles are magneticallycoupled to the second tractor trailer and to each other, and the firstpayload platform is coupled to the second trailer vehicle.
 10. Theapparatus as recited in claim 9, wherein the first trailer vehicle andthe first payload platform are coupled to each other by a firstrotatable coupling, while the second trailer vehicle and the firstpayload platform are coupled to each other by a second rotatablecoupling that is slidable relative to the first payload platform. 11.The apparatus as recited in claim 8, wherein the first payload platformcomprises a frame and an actuator supported by the frame, the firstmaintenance tool is supported by and movable relative to the frame, andthe actuator is configured to move the first maintenance tool relativeto the frame.
 12. The apparatus as recited in claim 9, furthercomprising a second payload platform coupled to the first and secondtrailer vehicles, and a second maintenance tool supported by and movablerelative to and along a length of the second payload platform, whereinthe second maintenance tool is one of the following: a non-destructiveinspection unit, a drill, a scarier, a grinder, a fastener, an appliquéapplicator, a ply mapper, a depainting tool, a cleaning tool, and apainting tool.
 13. The apparatus as recited in claim 12, wherein: thefirst trailer vehicle and the first payload platform are coupled to eachother by a first rotatable coupling; the second trailer vehicle and thefirst payload platform are coupled to each other by a second rotatablecoupling that is slidable relative to the first payload platform; thefirst trailer vehicle and the second payload platform are coupled toeach other by a third rotatable coupling; and the second trailer vehicleand the second payload platform are coupled to each other by a fourthrotatable coupling that is slidable relative to the second payloadplatform;
 14. The apparatus as recited in claim 8, further comprisingthird and fourth trailer vehicles which are magnetically coupled to thefirst payload platform and to each other.
 15. The apparatus as recitedin claim 8, further comprising further comprising a second payloadplatform coupled to the first trailer vehicle, and a second maintenancetool supported by and movable relative to and along a length of thesecond payload platform, wherein the second maintenance tool is one ofthe following: a non-destructive inspection unit, a drill, a scarfer, agrinder, a fastener, an appliqué applicator, a ply mapper, a depaintingtool, a cleaning tool, and a painting tool.
 16. The apparatus as recitedin claim 15, further comprising third and fourth trailer vehicles whichare magnetically coupled to the first payload platform and to eachother, and fifth and sixth trailer vehicles which are magneticallycoupled to the second payload platform and to each other
 19. A mobileplatform for performing a maintenance function on a skin of a hollowstructure comprising first and second skins connected by first andsecond vertical structural elements, the mobile platform comprising achassis, first and second trailer vehicles, and a maintenance toolsupported by and movable relative to the chassis, wherein the first andsecond trailer vehicles are magnetically coupled to the chassis throughthe first skin of the hollow structure and to each other through thefirst vertical structural element, wherein the chassis comprises: firstand second chassis parts coupled to each other, the first chassis partoverlying a first portion of the first skin disposed along the firstvertical structural element, the first maintenance tool being slidablycoupled to the second chassis part, and each of the first and secondchassis parts comprising a respective plurality of wheels in contactwith the external surface of the first skin; a drive wheel rotatablycoupled to the first chassis part and in contact with the externalsurface of the first skin; a first actuator mounted to the first chassispart for causing the first drive wheel to rotate; and a second actuatormounted to the second chassis part for causing the maintenance tool toslide along the second chassis part, wherein the magnetically coupledchassis and first and second trailer vehicles move in unison when thedrive wheel is rotated.
 20. The mobile platform as recited in claim 19,wherein the second actuator comprises a lead screw and a motor fordriving rotation of the lead screw, the axis of rotation of the leadscrew being transverse to a direction of travel of the chassis when thedrive wheel is rotated.